High-mu negative control grid velocity modulation tube



C. E. RICH Dec.8, 1964 HIGH-MU NEGATIVE CONTROL GRID VELOCITY MODULATION TUBE 2 Sheets-Sheet 1 Filed Aug. 20, 1962 BEAM INTERACTION REGION PULSE MODULATOR INVENTOR. CHARLES E. R/cH UUUUUUUV H UDUUUEDUDV WUUUUUUUUUV UUUUUUUDU UUUUUUUU 94..& 3,19

!I TTORNEY C. E. RICH Dec. 8, 1964 HIGH-MU NEGATIVE CONTROL GRID VELOCITY MODULATION TUBE 2 Sheets-Sheet 2 Filed Aug. 20, 1962 0.4 FRACTlON OF GRID OPENING INVENTOR. CHARLES E. R/CH United V States Patent HIGH- MU NEGATIVE CGNTRQL GRED VELGCITY MODELATION 'FUEL-E Charles E. Rich, Gainesviile, Fia., assignor to Sporty Rand Corporation, Great Necir, NE., a corporation of Delaware Filed Aug. 2d, 1962, Ser. No. 217,323 i 4 Claims. (Cl. 315-537) tion tube the control grid must perform the dual function of controlling the amount of current 'and preserving the precise electron beam shape required for the device to function properly. 'In normal pulsed operation of the control-grid tube, -the grid is biased to cut-off and a positive pulse is applied to the grid to drive the beam current to the required level. p In the pulsed operation of these tubes, the electron gun usually is designed so that the control grid lies alongua positive equipotential line in front of the cathcde 'and the maximum desired beam current is obtained when the 'grid potential is equal to the positive space potential between anode and cathode.

V Under these conditions there is no significant distortion of the beam due to the presence of the grid, and the electron trajectories in the beam may be de ermined by means known to those skilled in the art. lt has been found, however, that operating the control grid with 'a positive potential thereon has several unfavorable effects on the operation Off :the tube. The positive grid Will absorb electrons and will heat up, and this combined with the heat directly absorbed *from the cathode may be enough to cause the grid to become a primary emitter of electrons. Further difficulties arise When it is intended that the positive grid tubefaithfully reproduce an input grid driving pulse. This faithful reproduction is ditcult to 'achieve in a positive gridvtube because of the changing characteristics of the grid circuit during the application of a pulse that drives the grid positive.: In this situation, the normally cut-off grid will appear as a substantially infinite impedance, but when it goes into t the positive conductive region it draws current and presents a Variable positive impedance, and as its potential goes even more positive it Will enn't electrons and the impedance may make a violent excursion to a negative impedance. Additionally, when the grid potential passes through zero and into the positive region, the grid absorbs electrons from the beam which causes a notch to appear in the beam current characteristic curve. All of the above deleterious effects have made it impractical to produce high-gain negative control grid Velocity modulation tubes in beam power ranges much over 2 kilovolts average power.

The above-described difiicuities, are' avoided by operating the control grid always with a negative potential thereon,` or at least never allowing it to go more positive than the cathode potential. This avoids the changes in the characteristics of the grid circuit when the modulating pulse is applied thereto, largely avoids the bombardment and emission orf eleotrons by the grid, andthus permits operation at high power levels. Successful operation of the negative-grid Velocity modulation tube to V achieve high-gain characteristics at high power levels is t possible, in accordance with the present invention, by the novel control grid constructed and arranged in themanner to be described.' As operated in accordance with a "ice preferred embodiment of this invention the control grid having a mesh-type structure is at a negative potential 'with respect to the anode-cathode space potential and therefore each mesh has the effect of a converging lens 5 on the electron beam passing therethrough. As is well known, this ordinarily would give rise to undesirable aberration in the beam. This aberration may be reduced by properly spacing the grid elements with respect to r each `other and positioning the control grid closer to the emitting surface of the cathode in order that the portions of the emitting surface directly under the grid elements 'are cut off. This leaves as the effective emitting surface only those regions of the emitting surface directly under the open mesh of the grid, and relatively few of the electrons emanating from these regions have the components of Velocity parallel to the emitting surface that give rise to aberration. The grid is constructed to have a substantially uniform pattern, a uniform mesh for example,

so that the effect it has on the cathode etnitting surface is substantially similar throughout the emitting surface.

However, the control exerted by an ordinary and commonly used fine-wire grid on the electron beam decreases as the grid moves closer to the emissive suntace. Therefore, the step that -is 'intended to eliminate -the aberration ordi-narily would result in a loss of control by the grid on the beam current. This loss of control is overcome in accordance with the present invention by employing a deep grid in which the grid elements have apprecia'ble thickness in their dimension parallel to the electron beam axis and have predetermined spacings between themsolves. The deep grid is positioned so that the portion nearest the emitting surface will overcome the aberration in the manner described above, and the portion farthest removed from the emitting surface is sufiiciently spaced therefrom so that it will exercise adequate control over the electron beam.

It therefore is an object of this invention to provide a grid-controlled Velocity modulation electron beam tube having high gain and high average power rating.

lt is another object of this invention to provide a highgain grid-controlled Velocity modulation tube that is capable of operating with its control grid entirely in the negative potential region.

lt isa further object of this invention to provide a 4 high-gain grid-controlled -velocity modulation` tube in which the control grid is operated in the negative potential region and is constructed and arranged to provide a properly focused beam having a relatively high perve-ance and low abernation.

lt is another object of this invention to provide a Velocity modulation electron `beam tube that operates with -a negative controlFgrid and that is capable of achieving' 100% beam modulation with a grid drive of at least y as low as 2% of the beam voltage. 59 The invention will 'be more fully described by referring to the accompanying drawings Where-in: FTG. l is a simplified schematic illustration to be used "in describing an electron gun constructed in accordance with the present invention; 50 FTG. 2 is an illustration of a type of grid found suit- I able for use in the present invention, and;

FTG. 3 is a set of curves useful in explaining the operating principles employed in the present invention.

Referring now in more detail to FIG. 1, a cathode button ll@ is provided with a spherically-shaped electron emissive surface ll that provides electrons for the electron beam produced by the gun structure. A spherically-shaped of emissive surface ll and is comprised of spaced grid elements 15 Which extend into the plane of the paper and intersect the transverselyextending grid elements 16 electron permeable control grid 12 is positioned infront oa in a manner to form a mesh-type grid that has a substantially uniform pattern throughout its area. Grid elements 15 and 16 are supported in an annular mounting ring 17 in a conventional manner. PIG. 2 is a representation of a mesh-type of grid structure that has been found to be useful in the present invention. Other types of grids may be employed so long as the type of control exercised on the emitting surface is relatively uniform throughout the emitting surface. That is, each of the grid elements 15 and 16 cause only small portions of the emitting surface to be cut off, but this effect, or pattern, is similar throughout the entire emitting surface. An arch vane type of grid, for example, would not be satisfactory for the present purpose. An accelerating anode Zi) having an electron permeable grid 21 is axially positioned from control grid 12, as is common in electron gun construction. Anode 2f9 may be a gridless aperture if desired. A beam interaction region, which may be resonant cavities, or a slow wave propagating structure, occupies the region between accelerating anode 2E) and a beam collector electrode 25.

A voltage biasing source 26 maintains accelerating anode 2f3 and collector electrode 25 at a positive potential with respect to cathode E9. Control grid 12 is maintained at a negative potential with respect to cathode ft by means of a voltage biasing source 27, and input pulses are coupled to control grid 12 from a pulse modulator 28. Biasing source 27 is chosen to maintain the electron beam cut off during the quiescent stage of operation, i.e., absence of a modulating pulse from pulse modulator 23, and is sufficiently negative so that the control grid 12 never exceeds the potential of cathode lt) when a positive-going modulating pulse is coupled to grid 12 to turn on the electron beam.

As will be explained in more detail below and as illustrated in PIG. 1, each of the grid elements 15 and 16 has a predetermined width w and an axial depth d. The spacing between adjacent grid elements is indicated as m and the axial distance between emitting surface 11 and the top or farthest removed edge of the grid elements 1.5 and 16 is denoted by h. As will be explained, all of these dimensions are important considerations in the successful operation of a tube constructed and operated in accordance with the present invention.

As mentioned previously, the negative control grid positioned between the anode and cathode will cause each of the open meshes between the grid elements to act as a converging lens which causes defocusing of the beam due to aberration. Electrolytic tank studies were conducted on a negative control grid electron gun structure comprised of grid elements which intersected to form rectangular-shaped mesh areas of the type illustrated in PIG. 2, and with the control grid potential equal to cathode potential. These studies indicate that the aberration is caused mostly by electrons that leave the emitting surface at areas directly below the grid elements and the electrons that leave the emitting surface in the areas under the centers of the open meshes of the grid contribute least to the aberration. It also was found that the geometry of the grid mesh and its location to the emitting surface of the cathode affected the aberration. The curves plotted in FIG. 3 indicate the effect of the above factors on the aberration; the ordinate axis being calibrated in terms of aberration ratio B which is defined as the ratio of the electrical field component parallel to the emitting surface to the electrical field component normal to the emitting surface. The abscissa axis is calibrated in terms of the quantity F, known as the geometry factor which represents the ratio h/ m, wherein h and m are the dimensions illustrated in FIG. 1. The curves of FIG. 3 were obtained from measurements of the electrical field at the surface of the grid farthest removed from the emitting surface and were made for a grid having a given axial dimension d. Grids of different axial dimensions Will have similar families of curves, but the ordinate and abscissa Values Will be different. The zero point on the abscissa axis represents the central axis of one open mesh area of the grid and the 1.0 point on the abscissa axis represented a point directly under an adjacent grid elemerit. It is obvious from the curves of FIG. 3 that the aberration is least at the center of the open mesh areas of the grid and is greatest in the areas directly under the grid elements. Therefore, in accordance with this invention, the control grid 12'. of PIG. 1 is positioned sufiiciently close to the emitting surface 11 so that only the areas immediately under the grid elements are maintained cut off when the grid is driven to its least negative potential during the application of a modulating pulse thereto. This has the effect of eliminating from the electron beam those electrons that otherwise Would contribute most to cause aberration, and leaves only those electrons that have substantially only an axial component. Under ordinary circumstances, the presence of a fine-Wire grid this close to the emitting surface would greatly reduce the control of the grid on the beam current and would require a greatly increased negative bias to completely cut off the beam current. Both of these factors are functions of the geometric factor F, and in most applications of Velocity modulation tubes it is important that both of these factors be controllable within fairly close limits. The curves of FIG. 3 would seem to indicate that the smaller the value of geometric factor F the better the operation of the tube, since the aberration ratio B is minimum in this instance. This, however, is not necessarily true, because too small a value of F results in a poorly defined value of cut otf bias that is diflicult to repeat on a tube-to-tube basis in a roduction operation.

The loss of beam current control and the increase in grid bias necessary for cut off that result in positioning a commonly-used fine-wire control grid close to the emitting surface are overcome by increasing the depth d of the grid elements, this depth d being proportioned with respect to the spacing m between adjacent grid elements so that the portions of the grid elements farthest removed from the emitting surface are suffzciently separated therefrom to have adequate control over the electrons in the beam. As will be understood, all of the above-mentioned geometrical dimensions are interrelated with each other, and to an extent with the electrical parameter of the tube, so that in attempting to design a tube in accordance with the teaching of this invention it must be understood that exact dimensions are best derived empirically, as most often is the situation in the design of any Velocity modulation tube.

lt should be kept in mind that the grid potential becomes more effective and the anode less effective on the beam current as the depth d of the grid elements is increased, as the spacing m between adjacent grid elements is decreased, and as the distance h of the top surface of the control grid from the emitting surface is increased.

A tube constructed in accordance with this invention and operated with the grid potential never exceeding cathode potential had the following characteristics:

Perveance (grid voltage=cathode This tube had the following preferred physical dimensions and relationships for the grid structure:

Cross sectional dimensions of grid elements inches .O25 .008 Dimensions of open mesh area do .1195X.1195 Spacing of top of grid surface from emitting surface (h) inch .041

F a Spacing between adjacent grid elements a (m) inch .1275 Diameter of the grid inches 2.5 Potential gradient on the mesh opening center line as a fraction of the gradient of a diode of the same dimensions .62

Acceptable operation of a negative grid modulation tube constructed in accordance with this invention may be achieved with Values of h/m ranging from .25 to .50, and with ratios of d/m ranging from .10 to .25. The tube described above achieved substantially 100% beam modulation with a grid drive of 2% of the beam voltage. It was found that to operate the tube in the desired perveance range of 2 10- the geometry factor F should be less than 0.4.

Tubes constructed in accordance with the present invention have operated'. successfully as high-gain highpower pulsed amplifiers and have provento be superior to similar types of tubes operated 'with a positive grid in these respects:

(a) The negative grid draws no current and therefore requires no power to operate.

(b) A high average beam power requirement may be easily met. i

(c) There is no -disturbance due to non-linear loading effects on the grid and no intercept note in the beam current during the occurrence of a modulating pulse. It is possible, therefore, to drive the grid through a coupling transformer which also may serve as the electrical isolation from ground, 'thus eliminating the need for floating bias modulators. r

While the invention hasbeen described in its preferred embodiments, it is to be understood that the words which have been used are words of description rather than of limitation and that changes within the purview of the appended claims may be made ,without departing from the true scope and spirit of the invention in its broader aspects.

What is claimed is:

l.' An electron beam gun for forming a longitudinally extending homogeneous and axially symmetric electron beam, said gun comprising an electron emissive cathode for emitting electrons directed along an axial beam,

an apertured anode electrode axially positioned from said cathode for attracting said electrons and forming them into a shaped beam for passage through said anode,

an electrode permeable beam control grid positioned intermediate .said cathode and anode in the path of said electron beam, said control grid being electrically biased in the quiescent state at a potential below the potential of said cathode surface and at` all times during operation having a potential thereon so as not to collect electrons fromsaid beam,

said grid being comprised of intersecting grid elements extending in a uniform manner through said beam and the type of control exercised by a portion of said elements on a respective portion of said emitting surface' being substantially the same type of control' as exercised by other portions of said grid elements on other portions of said emitting surface,

said control grid being positioned sufficiently close to said cathode surface to maintain said beam substantially completely cut-oit with said quiescent bias voltage applied thereto and to maintain only the emitting surfaces immediately under the grid elements cut-off with the full operating potential applied thereto,

said grid elements having a depth in the direction along said axis and having a spacing therebetween to accomplish 100% current modulation of said beam with a grid voltage swing of approximately 2% of the potential difference between said anode and said cathode surface.

2. A high-gain Velocity modulation electron beam tube having a control grid and adapted for pulsed operation with the control grid signal never more positive than the cathode potential of said tube, said tube including,

an electron emitting cathode surface for emitting' electrons in a beam directed along an axis, e v p means including an anode eleetrode anda Collector electrode successively positioned along said axis from said cathode surface and electrically biased at potentiais suitable for forming said beam and for directiug said electrons along said axis,

said control grid being of intersecting grid elements that form substantially rectangularly-shaped meshes that are positioned in the path of said beam,

said control grid being electrically biased in the quies cent state to substantially completely suppress electron flow in said beam and during the application of a signal thereto never to be more positive than the potential of said cathode surface,

during the application of a signal thereto, said grid elements maintaining below cut-off only the areas of the emitting surface immediately therebelow While the areas of the emitting surface opposite the open mesh areas of said grid emit electrons in a direction along said axis,

the surfaces of said grid elements furthest removed from said emitting surface being spaced therefrom by a distance h and adjacent grid elements being spaced from each other by a distance m :and the ratio of h to m varies from .25 to .50, and the ratio of depth d of said grid elements along said axis to the gridelement spacing m varies from .lO to .25.

3. A Velocity modulation electron beam tube comprising,

a cathode having an electron emitting surface thereon,

means including an apertured anode and a Collector electrode successively axially spaced from said cathode to form said electrons into a homogeneous beam that is symmetrical about a longitudinally extending axis,

an electron beam interaction structure intermediate said anode and Collector for permitting said beam to interact with electromagnetic energy present in said structure,

an electron permeable control grid positioned in the path of said beam intermediate said emitting surface and said anode for controlling the current in said electron beam,

said controd grid being comprised of intersecting grid the surfaces of said grid elements furthest removed from said emitting surface being spaced therefrom by a distance h and adjacent grid elements being spaced from each other by a distance m, and the ratio of h to m being about .32, and the ratio of depth d of said grid elements along said axis to the gridelement spacing m being about .19.

4. A Velocity modulation electron beam tube comprisa cathode having an electron emitting surface thereon,

means including an apertured anode and a Collector electrode successively axially spaced from said cathode to form said electrons into a homogeneous beam t 7 that is symmetrical about a longitudinally extending axis,

an electron beam interaction structure intermediate said anode and collector for permitting said beam to interact With electromagnetic energy present in said structure,

an electron permeable control grid positioned in the path of said beam intermediate said ernitting surface and said anode for controlling the current in said electron beam,

said control grid being comprised of intersecting grid elements that form substantially rectangularly-shaped meshes in the path of said beam,

said control grid being electrically biased in a quiescent state to substantially completely suppress electron flow in said beam and during the application of a modulating signal thereto never to be more positive than the potential of said cathode,

during the application of said modulating signal thereto said grid elements maintaining below cut-off only 8 the areas of emitting surface directly therebelow while the areas of the emitting surface directly be low the open meshes of said grid emit electrons in a direction along said axis, the dimensions of said grid elements in the direction parallel to said beam exceeding the dimensions transverse thereto by an amount to permit 100% beam current modulation with a grid signal whose magnitude is less than 5% of the anode-cathode potential.

References Citer! in the file of this patent UNITED STATES PATENTS 2,392,379 Hansen Jan. 8, 1946 2,451,249 Smith et al. Oct. 12, 1948 2,825,842 Kenyon Mar. 4, 1958 3,021,447 Jepsen et al Feb. 13, 1962 FOREIGN PATENTS 849,734 Germany Sept. 18, 1952 

1. AN ELECTRON BEAM GUN FOR FORMING A LONGITUDINALLY EXTENDING HOMOGENEOUS AND AXIALLY SYMMETRIC ELECTRON BEAM, SAID GUN COMPRISING AN ELECTRON EMISSIVE CATHODE FOR EMITTING ELECTRONS DIRECTED ALONG AN AXIAL BEAM, AN APERTURED ANODE ELECTRODE AXIALLY POSITIONED FROM SAID CATHODE FOR ATTRACTING SAID ELECTRONS AND FORMING THEM INTO A SHAPED BEAM FOR PASSAGE THROUGH SAID ANODE, AN ELECTRODE PERMEABLE BEAM CONTROL GRID POSITIONED INTERMEDIATE SAID CATHODE AND ANODE IN THE PATH OF SAID ELECTRON BEAM, SAID CONTROL GRID BEING ELECTRICALLY BIASED IN THE QUIESCENT STATE AT A POTENTIAL BELOW THE POTENTIAL OF SAID CATHODE SURFACE AND AT ALL TIMES DURING OPERATION HAVING A POTENTIAL THEREON SO AS NOT TO COLLECT ELECTRONS FROM SAID BEAM, SAID GRID BEING COMPRISED OF INTERSECTING GRID ELEMENTS EXTENDING IN A UNIFORM MANNER THROUGH SAID BEAM AND THE TYPE OF CONTROL EXERCISED BY A PORTION OF SAID ELEMENTS ON A RESPECTIVE PORTION OF SAID EMITTING SURFACE BEING SUBSTANTIALLY THE SAME TYPE OF CONTROL AS EXERCISED BY OTHER PORTIONS OF SAID GRID ELEMENTS ON OTHER PORTIONS OF SAID EMITTING SURFACE, SAID CONTROL GRID BEING POSITIONED SUFFICIENTLY CLOSE TO SAID CATHODE SURFACE TO MAINTAIN SAID BEAM SUBSTANTIALLY COMPLETELY CUT-OFF WITH SAID QUIESCENT BIAS VOLTAGE APPLIED THERETO AND TO MAINTAIN ONLY THE EMITTING SURFACES IMMEDIATELY UNDER THE GRID ELEMENTS CUT-OFF WITH THE FULL OPERATING POTENTIAL APPLIED THERETO, SAID GRID ELEMENTS HAVING A DEPTH IN THE DIRECTION ALONG SAID AXIS AND HAVING A SPACING THEREBETWEEN TO ACCOMPLISH 100% CURRENT MODULATION OF SAID BEAM WITH A GRID VOLTAGE SWING OF APPROXIMATELY 2% OF THE POTENTIAL DIFFERENCE BETWEEN SAID ANODE AND SAID CATHODE SURFACE. 